Radio-frequency (RF) signals generated at a mobile handset generally are amplified, transmitted through a handset antenna and sent to a base station for distribution to receivers. Often the frequency bands of operation of the handsets are predetermined, mainly in the frequency range from 450 MHz to 2.6 GHz for various mobile standards such as WCDMA (wide band code division multiple access) and CDMA (code division multiple access).
In general, the handset is required to transmit at a high output power level when it is farther away from a receiving base station in order to maintain a predetermined signal strength at the base station for sufficient reception. Conversely, the closer the handset to the base station, less transmitted power would be required. A handset's output power is adjusted using the command embedded within the RF control signal transmitted from the base station to the handset.
The handset transmitted signal, and hence the RF power amplifier output signal, has to meet the government regulations on spectral re-growth (also known as linearity—often measured in terms of adjacent channel leakage power ratio (ACLR) which stipulates the maximum allowable interference to other frequency channels in order to minimize interference between signals).
Some known mobile devices (handsets) have RF power amplifiers powered by the full battery voltage at all times. RF power amplifiers are generally designed to meet the linearity specification at maximum transmit power level (e.g., +28 dBm in certain WCDMA mobile handsets) under such a bias condition. Statistically, power amplifiers transmits at maximum linear output power only for a small fraction of time, while most of the transmissions take place at a considerably lower power levels (10-20 dB below maximum power).
The actual output power level from the power amplifier (and hence the handset), is continuous from some −50 dBm to 28 dBm. Multi-gain state power amplifiers, compared to conventional single-path power amplifiers, consume less current at low power outputs.
Multi-gain state power amplifiers are commonly implemented with two or three power gain states. In a three gain state solution the three states include high power (HP), medium power (MP), and low power (LP). For an example normal WCDMA waveform, the HP gain state might be set to a desired max output power range of 16 dBm to 28 dBm, the MP gain state might be set from 8 dBM to 16 dBM, and the LP gain state might be set for all power levels below 8 dBm. In short, multi-gain state power amplifiers are implemented with two, three (or more) power paths, each for delivering a certain predetermined max output power at a fixed gain. A switch point (in dBm) is value which defines a transition power level at which the handset is to switch (jump) from one PA gain state to the one above it or below it. At present, multi-gain state PAs do not dynamically change this PA switch point to account for inherent differences of transmitted waveform, such as linearity differences.
As mentioned, spectral re-growth can occur when the PA is forced to operate in a non-linear region, which occurs when driving the PA near its 1 db compression point. Spectral re-growth therefore describes the increase in out-of-band signal energy at the power amplifier output due to non-linear amplifier effects. Spectral re-growth is most pronounced within a channel adjacent to a desired transmit channel. For UMTS, the requirement on the power amplifier is defined by an Adjacent Channel Leakage Ratio (ACLR) at +/−5 MHz of the desired channel. The voltage gain characteristic of the power amplifier may be characterized as follows:vo(t)=g1·vi(t)+g2·vi(t)2+g3·vi(t)3+ . . . +gn·vi(t)n  Equation (1)where g1·vi(t) is the linear gain of the amplifier and the remaining terms (i.e., g2·vi(t)2+g3·vi(t)3+ . . . +gn·vi(t)n) represent the non-linear gain. If the signal carries a modulated third Generation Partnership Project (3GPP) Radio Frequency (RF), then the non-linear terms will be generated as a result of inter-modulation distortion, resulting in in-band distortion terms causing an increase in Error Vector Magnitude (EVM) and out-of-band distortion causing an increase in ACLR.
Multi-code signals, such as those in UMTS Release 5, 6 and 7, as well as those in certain LTE specifications (e.g., 3GPP Release 8), exhibit an increase in peak-to-average power resulting in larger dynamic signal variations. These increased signal variations require increased amplifier linearity, resulting in increased power consumption. Recent results have shown that it is not efficient to directly transfer dB for dB (i.e., the ratio of peak power to average power of a signal, also known as peak-to-average ratio (PAR)) to amplifier power reduction. Analysis of the amplifier spectral re-growth has shown that the 3rd order non-linear gain term (“cubic gain”) is the dominant cause of ACLR increase. The total energy in the cubic term is dependent on the statistical distribution of the input signal.
With the introduction of High Speed Uplink Packet Access (HSUPA), a new method of estimating amplifier power reduction called the Cubic Metric (CM) was introduced in Release 6. The CM is based on the amplifier cubic gain term. The CM describes the ratio of the cubic components in the observed signal to the cubic components of a 12.2 kbps voice reference signal. The CM applies to both High Speed Downlink Packet Access (HSDPA) and HSUPA uplink signals. Statistical analysis has shown that the power de-rating based on an estimation of the CM exhibits a significantly smaller error distribution when compared to power de-rating based on 99.9% PAR, where the error distribution is the difference between the actual power de-rating and the estimated power de-rating.
3GPP specifies a Maximum Power Reduction (MPR) test which verifies that the maximum transmit power of a mobile handset is greater than or equal to the nominal maximum transmit power less an amount herein termed “maximum-MPR,” where maximum-MPR is a function of the transmitted signal's CM.
A handset must know the value of CM in order to compute the selected MPRs and, if required, (i.e., if the handset is operating at near maximum power), ultimately use this information to actually set the transmit power at a transmit power level which is backed off from max power level by an amount equal to MPR. Even if the receiving base station cannot receive the transmit signal at this lower (backed off) transmit power level, the standard allows for the handset to transmit at lower power levels. Because the PA is already at the highest gain state and max output power level, it cannot switch up to any next higher gain state or power level.
Any multi-code signal (characterized by the physical channels being transmitted, their channelization codes and weights called β terms) has its particular CM and PAR. In UMTS, the signal, and thus the CM and PAR, can change every 2 or 10 msec Transmit Time Interval (TTI). It can be shown that for Release 6 UMTS there are over two hundred thousand combinations of physical channel parameters and quantized β terms; each such combination is herein termed a possible signal. It is therefore necessary for a handset to look up CM or PAR (by way of a look up table or the like) or either measure or estimate these values within some allowable error from the signal's characteristic parameters. Nevertheless, measuring CM or PAR from an actual signal is well known.
Voice waveforms are typically associated with high linearity, which means that any associated linearity metric (e.g. cubic metric, PAR, etc) is high relative to data waveforms. Data waveforms, on the other hand, have a large range of linearity metrics.
Existing PA implementations do not distinguish between voice and data waveforms, or between more and less linear waveforms to adjust the switch point to account for the linearity difference between them. This unfairly results in both voice and data waveforms being switched based on predetermined switch points independent of any linearity characteristic of the underlying transmit waveform. When a lower gain state would be more optimum at a predetermined transmit power level, battery resources are wasted.
Multiple vendors offer competing multi-gain state PA solutions. A handset integrator selects a best solution for a given reference design thus being free of the burden of designing its own PA solution. One disadvantage of this is that the PA may not be completely optimized to provide optimum power utilization. For example, PA solutions configured for multi-mode, multi-media capable mobile device applications, may be optimized for voice instead of data usage, or at less than optimum power transition levels for a particular phone configuration. On the other hand, existing off-the-shelf solutions being low-cost, already proven solutions that are easy to integrate, the trade-off in efficiency is oftentimes an acceptable or only cost effective option for an integrator.
Table 1 below illustrates example PA characteristics of an example multi-gain state PA designed for use in a WCDMA mobile device. The example multi-gain state is designed to operate across the three PA gain states, each of which is associated with a specific maximum power output level and a specific gain value.
TABLE 1CharacteristicConditionValueGainHigh Power Gain State (HP)26dBMaximum Output Power = 28.0 dBmMid Power Gain State (MP)16dBMaximum Output Power = 16.0 dBmLow Power Gain State (LP)15dBMaximum Output Power = 8.0 dBmTotal SupplyHigh Gain State465mACurrentMaximum Output Power = 28.0 dBmMid Gain State72mAMaximum Output Power = 16.0 dBmLow Gain State31mAMaximum Output Power = 8.0 dBmQuiescent CurrentHigh Gain State97mAMaximum Output Power = 28.0 dBmMid Gain State14mAMaximum Output Power = 16.0 dBmLow Gain State11mAMaximum Output Power = 8.0 dBm
As can be seen from Table 1, operating the multi-gain state PA at a lower possible gain stage consumes significantly less current. Hence, a baseband processor—which typically sets the operating state of the PA during phone operation—must select the best power gain state to set the PA to in order to optimize power consumption without compromising transmit signal degradation, adjacent channel leakage, and the like either required by the network operator, and/or required to meet the technical specifications of the product as defined by an associated standard specification, or government imposed signal interference rules and regulations.
In certain wireless communication protocols, such as WCDMA for example, a handset or like portable device must be able to switch, based on predetermined switch points, to the appropriate output power level and specific gain state.
A baseband processor in the handset typically determines and then sets the gain state of the three gain state PA by toggling a two bit digital input (e.g., IC pins Vmode0 and Vmode1). A two gain state PA device may require a one bit digital input instead.
In WCDMA, the baseband processor may perform rate selection to control or reduce power leakage to the adjacent channel that can be caused by the handset. Rate selection involves selecting a data rate and coding scheme for the intended signal transmission or burst as a way to control PA response.
The baseband processor programs the handset transmitter circuitry so as to set up the necessary transmitter configuration to enable the uplink transmission. Transmitter configurations can be characterized by one or more channels in the code, frequency, or time domain or any combination thereof and may include other attributes such as channel modulation type.
An uplink transmitter configuration for WCDMA may be characterized by two or more code channels with differing modulation, spreading factor, channelization code, and I or Q branch assignments. For example, a multi-channel WCDMA uplink transmission might include (i) a dedicated physical control channel (DPCCH), (ii) a dedicated physical data channel (DPDCH), (iii) a high speed dedicated physical control channel (HS-DPCCH), (iv) an enhanced dedicated physical control channel (E-DPCCH), and (v) an enhanced dedicated physical data channel (EDPDCH), to support a single transmission link event.
In a multi-mode device capable of supporting newer, as well as older, releases of a wireless protocol, as well as multi-mode devices capable of supporting other wireless protocols (e.g., CDMA 1x, CDMA2000, OFDM, etc.), the number of possible transmitter configurations can be quite large.
It is known that a given transmitter configuration is likely to cause a PA to behave more or less linear than another transmitter configuration. For this reason, protocols, such as WCDMA, require the baseband processor to assume a worst case peak to average power response for a given uplink transmission, and to account for this by requiring the handset to account for MPR, as previously explained.
In terms of signal gain, there might be very little actual power gain difference between a lower gain state and the next higher gain state. Referring to Table 1, the difference in actual power gain (i.e., Pout/Pin) between low and mid gain states is only 1 dB for example.
Once the necessary transmitter configuration is set, the waveform characteristics of an uplink transmit signal can vary depending on the type of signal being transmitted.
The main problem with a multi-gain state PA is the difficulty (in terms of high design costs) to change bias and reference voltages to set the PA to account for non-linearity variations throughout its transmit power dynamic range.
One known approach to account for non-linearity variations generally is to utilize a single gain state PA which is externally driven by a variable switch mode power supply (SMPS). An SMPS varies the voltage supplied to the PA more accurately over a desired transmit dynamic range, thus improving battery referred current due to the efficiency of the SMPS. An SMPS is a bulky, expensive component and for this reason is intentionally avoided. Also, using an SMPS requires calculating the associated linearity metric for each waveform on the fly, and generating appropriate signals to program the SMPS, in order to set any bias, reference, and the like PA settings which will in turn tune the PA to a desired gain state.
It is highly desirable to be able to more efficiently switch between gain states of a multi-gain state PA to account for waveform non-linearity characteristics.
To facilitate understanding, identical reference numerals have been used where possible to designate identical elements that are common to the figures, except that suffixes may be added, where appropriate, to differentiate such elements. The images in the drawings are simplified for illustrative purposes and are not necessarily depicted to scale.
The appended drawings illustrate exemplary configurations of the disclosure and, as such, should not be considered as limiting the scope of the disclosure that may admit to other equally effective configurations. Correspondingly, it has been contemplated that features of some configurations may be beneficially incorporated in other configurations without further recitation.